Drive shaft and rotor hub for helicopter flexible rotor system

ABSTRACT

A rotor system according to this invention includes a structurally flexible rotor shaft for transmitting rotor torque and other rotor loads to a rotor hub. The rotor hub is adapted to have rotor blades mounted thereon and is mounted by an elastomeric spherical bearing whose center is located at the rotor center. A flexible shaft made from fiber reinforced resin matrix material, connected to the rotor shaft at a connection located below the bearing center, extends vertically from that location through the bearing to a position located above the bearing center where it is connected to a connecting member fixed to the upper surface of the rotor hub. 
     The flexible shaft is structurally stiff with respect to the mode in which it transmits rotor torque compared to the rotor torque stiffness of the other components. However, the bending stiffness and axial stiffness of the flexible shaft is substantially less compared to the mode in which rotor moments and forces are transmitted from the other components to the rotor shaft.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to the rotor hub, rotor blades, driveshaft and controls for a helicoptor rotor by means of which the aircraftis supported aerodynamically, controlled for flight maneuvers andconnected to the drive system.

This invention is particularly suited for use with helicopter rotorsystems wherein conventional, rigid mechanical bearings, by which thedynamic rotor components are usually interconnected, are replaced byflexible, elastomeric bearings that provide resilient, dimensionallyvariable interconnections among the various components.

2. Description of the Prior Art:

A helicopter rotor includes a rotor hub driveably connected on the endof a rotor shaft, which transmits power through a transmission from theengines. Rotor blades are mounted on the hub, spaced angularly from oneanother and extending radially outward from the axis of the hub. Therotor blades are formed with an airfoil contoured surface so thataerodynamic lift forces are produced as the blades rotate through theatmospheric air. The lift force and a vertically directed component ofthe centrifugal forces of the rotor blades operate to support theaircraft and horizontal or forward directed components of theaerodynamic lift and centrifugal forces operate to accelerate andmaneuver the aircraft. The magnitude of the lift force varies as afunction of the velocity of the rotor blades relative to the ambient airand the angle of attack or pitch angle of the blades relative to theairstream.

For a particular blade pitch angle, the lift developed by a rotor bladeis greater as it advances due to rotation toward the airstream createdby the forward speed of the aircraft than the lift developed as theblade retreats due to rotation from the airstream. The velocity of theairstream relative to the advancing blade is greater than its velocityrelative to the retreating blade because the speed due to rotation addsto this speed due to forward flight, but as to the retreating blade,these speeds are opposed. To accommodate the speed difference and tomaintain the lift force more nearly equal on the blades around theentire rotational path, the rotor controls increase the pitch angle onthe portion of the rotor where the speeds are subtractive and decreasethe pitch angle where the speeds are additive.

In conventional rotor systems the pitch angle change is made by pitchlinks fixed at one end to a gimballed pitch ring, which is tilted withrespect to the rotor hub, and fixed at the opposite end to a pitch arm,which is connected to the root of the rotor blade eccentric of the bladepitch axis. The blade pivots about the pitch axis in accordance with thetilt of the pitch ring because the blade is pivotably connected to thehub by a torsionally flexible tension-torsion pack through which thecentrifugal force is carried rigidly to the hub.

The rotor blades also flap about horizontal axes located at the bladeroot so that bending moments about the flap axis are controlled to anacceptable magnitude. The flapwise pivoting conventionally occurs bymounting the blade on bearings supported on a horizontal pin carried onthe rotor hub. This pinned connection, located at a predetermineddistance from the rotor center, assures that the flapwise moment at theconnection is zero and establishes the magnitude of the rotor moment atthe rotor center as the product of the perpendicular force on the rotorarm times the distance from the rotor center to the flapwise pinnedconnection.

The rotor blades also pivot about a vertical axis located apredetermined radial distance from the rotor center. In conventionalrotor systems a damper is connected across this to tune the rotor systemagainst latent instabilities that can arise due to starting the rotorwhile the helicopter is elastically supported on ground wheel oleos, acondition called ground resonance. The presence of the vertical orlead-lag axis assures that the moment at the connection and at the rotorcenter is controlled to an acceptable magnitude.

Recently, more advanced rotor systems have substituted structuralflexibility for the mechanical pinned rotor blade connections at itsroot end attachments to the rotor hub about the flapwise axis, thelead-lag axis and the pitch axis, while maintaining the rigid forcecontinuity essential to proper operation. For example, instead of thehorizontal bearing connection the rotor blade has been formed with amember, located near the rotor, having relatively low flapwise bendingstiffness and high axial or spanwise tension stiffness. This member,called a flexure carries all of the blade loads to the rotor yet itpermits the blade to pitch, flap and lead-lag without providing bearingconnections to the rotor hub to control the magnitude of the rotormoment.

In rotors of this kind, the flexure is made from fibers such as fiberglass, Kevlar, graphite, Boron, etc. supported and connected bypolymeric resin matrices of materials such as epoxy, phenolic,polyesters, etc. However, although the flexure is flexible with respectto flapwise bending, edgewise bending and torsion, it must have adequatestrength to transmit flapwise bending moments, edgewise or lead-lagbending moments, blade pitch or torsion moments and centrifugal forcefrom the blade to the rotor hub.

Furthermore, the flexure cannot duplicate entirely the effect of theconventional pinned connections of the rotor blade to the rotor hubbecause of the structural continuity that must be maintained. Therefore,the flapwise bending moment in particular is larger than if a flapwisepinned connection were used. Accordingly, the rotor hub moments aregenerally higher for the advanced rotor hub systems that employstructural rotor blade flexibility as a means to simulate the effect ofconventional pinned rotor blade support on the rotor hub.

When the rotor blade root end bending moments are large the rotor momentis large and requires additional control force to adjust the attitude ofthe helicopter rotor with respect to the attitude required for amaneuver or flight speed change.

A rotor system of the flexural type is described in the paper entitled"Flexible Matrix Composite Applied to Bearingless Rotor Systems", byA.J. Hannibal et al, presented at the American Helicopter SocietyComposite Structures Specialists Meeting in Philadelphia, Pennsylvania,March 1983. U.S. Pats. No. 3,669,566 and 4,332,525 describe helicopterrotor systems that employ composite materials.

SUMMARY OF THE INVENTION

To overcome the difficulties associate with large rotor moments, whicharise with structurally flexible, bearingless rotor blades, a rotorsystem according to this invention includes a structurally flexiblerotor shaft for transmitting rotor torque and other rotor loads to arotor hub. The rotor hub is adapted to have rotor blades mounted thereonand is mounted by an elastomeric spherical bearing whose center islocated at the rotor center. A flexible shaft made from fiber reinforcedresin matrix material is connected to the rotor shaft at a connectionlocated below the bearing center, extends vertically from that locationthrough the bearing to a position located above the bearing center whereit is connected to a connecting member fixed to the upper surface of therotor hub.

The flexible shaft is structurally stiff with respect to the mode inwhich it transmits rotor torque compared to the rotor torque stiffnessof the other components. However, the bending stiffness and axialstiffness of the flexible shaft is substantially less compared to themode in which rotor moments and forces are transmitted from the othercomponents to the rotor shaft.

It is an object of this invention to provide a rotor system having aflexible support between the rotor shaft and rotor hub so the angularflapwise movement of the rotor occurs with little restriction externalto the rotor. The elastomeric bearing and flexible shaft provide aflexible support for the rotor that permits relatively large angularmovement about the center of the bearing.

It is another object of this invention to provide a relatively stiffstructural load path between the rotor shaft and rotor hub for loadsother than those that cause tilting of the rotor about the bearingcenter. The nature of the fiber orientation and the resin matrix thatsupports the fibers of the flexible shaft operates to produce therequisite torsional stiffness and flexible bending stiffness to realizethese objects.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side cross section taken at a vertical plane through therotor center of a flexible rotor system according to this invention.

FIGS. 2A and 2B are graphs that show the variations of Young's Modulusand shear modulus with winding angle for epoxy and urethane resinmaterials supporting E fiber glass material.

FIG. 2C shows the variation of the ratio of Young's modulus to shearmodulus with winding angle for epoxy and urethane resin matricessupporting S fiber glass material.

FIG. 3 is a side cross section taken at a vertical plane through therotor center of a flexible rotor system in which a flexible matrixdiaphragm coupling is used.

FIGS. 4A and 4B show the flexible matrix diaphragm coupling in deflectedpositions.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, the rotor hub 10 is adapted to have rotorblades 12, 14 mounted thereon and extending radially outward from theaxis 16, which is directed substantially vertical from a transmissionoutput. A rotor shaft 18 is driveably connected to the transmissionoutput and is connected by an extension 20 to the inner race 22 of aspherical elastomeric bearing whose center is at 24. The external race26 is connected to hub 10. Located between the inner race and outer raceis a bearing core comprising multiple laminates of elastomer alternatingwith and bonded to metal laminas. All of the laminas and both races ascentered at 24. The bearing races are arranged in pairs located onopposite vertical sides of the blade pitch axes 30 and 32. The bearingprovides a resilient connection between the races and permits rotationbetween the races about the entire center 24 that is unrestricted exceptfor the shear stiffness of the elastomeric center.

The rotor blades include pitch elements 34, 36 made from fiberreinforced with matrix composite material wherein the fibers aredirected predominately parallel to the pitch axes. The pitch elementsare fixed to the rotor hub 10 to which the centrifugal force and otherrotor blade loads are tranferred. The torsional stiffness of the pitchelements is very low compared to its axial stiffness and bendingstiffnesses about the flapwise and edgewise axes because the fibers aredirected parallel to the pitch axes.

Hub 10 surrounds the rotor center and is connected mechanically to theouter races 26 of the bearing. Each blade is fitted with a pitch cuff38, a tube located over each pitch element and extending spanwise alongthe pitch axis from an attachment to a pitch link at its inboard end toan attachment to the blade located outboard of the outer end of thepitch element. The pitch cuff is made from fiber reinforced polymerresin wherein the fibers are biased or angularly turned with respect tothe pitch axis. Because the pitch cuff should have a large torsionalstiffness compared to its bending and axial stiffnesses, the fibers arepreferably biased approximately 45° in both angular directions onopposite sides of the pitch axis.

A flexible shaft assembly 40 is connected to rotor shaft 18 below therotor center, extends upward past the rotor center, and is connected at42 to connecting member 44. The connections of the flexible shaftassembly to the rotor shaft and connecting members can be mechanicalconnections made by bolts directed radially through the shafts, orbonded connections made by polymerizing epoxy or another materiallocated between the surfaces to be joined. Alternatively, theconnections can be made as described in pending patent application Ser.No. 706,242, entitled "Coupling and Method of Connecting Two Shafts Withthe Coupling" by William Rumberger assigned to the assignee of thepresent invention.

The flexible shaft 40 can comprise an outer flexible shaft 46 and aninner flexible shaft 48 concentrically arranged and extending verticallyalong the same distance from the rotor shaft 18 to the connecting member44. Concentric shafts 46 and 48 are spaced from one another radially bya layer of filler 50 which can be an epoxy bondline formed after thetubes are manufactured.

The connecting member 44 can be made from metal such as alloy steel ortitanium or it can be formed of the same material as is used for theflexible shafts. In either case connections are made at 52 between theupper surface of the rotor hub and the radial flange 54 of member 44.

The lower rotor controls 56 include a link 58 connected to a pitch arm60 that extends outward from a connection to the rotor hub and centeredat 24. As pitch arm 60 is moved between the positions shown in FIG. 1the attitude of the rotor hub can be altered in relation to thefuselage. In this way the direction of the rotor lift is changed inaccordance with manual control input by the pilot as required for flightmaneuvers and forward speed flight. The slider guide 62, mountedslidable on and coaxially with the rotor shaft, is connected at itsupper end to a pitch ring 64 on which the lower end of the pitch links(not shown) are connected. As the slider is moved by the lower controlsupward and downward the pitch links transfer that motion to the rotorblade through the pitch cuff to vary the blade pitch angle. A cover 68encapsulates the rotor.

The rotor is gimballed by the bearing about rotor center 24 andconnected to the rotor shaft. The rotor and blades rotate about center24 and transfer the rotor moment from rotor shaft 18 through the bearingto a rotor hub 10. The hub moment causes the rotor to tilt around center24 and flexible shaft assembly 40 bends with relatively littleresistance about a bending axis perpendicular to its longitudinal axisto the deflected position shown at 66. The flexible shafts offer littlebending resistance in comparison to the stiffness of the load path fromrotor shaft 18 to rotor hub 10 through the bearing. Similarly, flexibleshaft assembly 40 offers little resistance to force directed along itsaxis compared to the stiffness of the load path from rotor hub 10, andthe bearing to rotor shaft 18. However, the torsional stiffness of theflexible shaft assembly is large relative to the torsional stiffness ofthe load path from rotor shaft 18 to rotor hub 10 through the bearing.Accordingly, substantially all the rotor torque carried by rotor shaft18 from the transmission output is transferred to the flexible shaftassembly 40 at the connection located below the rotor center 24. Therotor torque is then transferred from flexible shaft assembly 40 to theconnecting member 44 at connection 42 and then is transferred to rotorhub 10 at the mechanical connection 52.

To provide the torsionally stiff and flexurally soft structuralcharacteristics, the flexible shafts are formed from S-glass or E-glassfibers reinforced with flexible urethane polymers such as MXU-34,MXU-17, MXU-10 or commercial urethane polymers #1 and #2 available fromLord Corporation, 1635 West 12th Street, Erie, Pa. 16512. Thesematerials and the configuration of the fibers were discussed in thetechnical papers entitled "Progress in the Development of ElastomericMatrix Composites", by D.P. McGuire et al, presented at the AmericanHelicopter Society Composites Manufacturing Specialists Meeting,Stanford, Connecticut, June 1985 and the technical paper by A.J.Hannibal et al. previously cited. The entire disclosure of thesetechnical papers is incorporated herein by reference.

FIG. 2A shows that Young's Modulus for E-glass fibers supported by amatrix of urethane polymer is substantially lower than for epoxy polymerand that the lowest Young's Modulus occurs when the fibers are directedat 45° on both sides of the longitudinal axis of the tube. FIG. 2B showsthat the shear modulus, which is a measure of the torsional stiffness ofthe tube, is a maximum when the fiber bias angle with respect to thetube axis is 45° and that urethane matrix material have only slightlylower shear modulus than epoxy matrix materials. FIG. 2C shows that theratio of Young's Modulus to Shear modulus can vary according to fiberdirection from 0.001 to 10,000 for urethane matrix materials compared toa range of 0.50 to 30 for epoxy materials.

Preferably the shafts 46 and 48 that comprise the flexible shaftassembly 40 and the connecting member 44 are formed from urethane basedmatrix material supporting fibers of S-glass, E-glass, graphite orKevlar and the fibers are turned angularly 45° with respect to the axisof shafts 46 and 48 and member 44.

Referring now to FIG. 3, the rotor shaft 18', is connected to the innerrace 70 of a bearing 72 having a center at 24. The outer race 74 of thebearing is connected to a torsionally rigid coupling 76, which surroundsbearing 72 and makes a stiff torsional connection between its connectionat 78 to the outer bearing race and its connection at 80 to the driveplate 82.

The coupling is made also from urethane matrix material, preferablyMUX-34, MUX-17,or MUX-19 and S-glass, E-glass, Kevlar or graphite fibersdirected at a 45° degree bias relative to the axis of rotor shaft 18'.The folds 86, 88, 90 facilitate movement of the coupling to positions,such as those of FIG. 4A and 4B, that accommodate movement of the rotorhub and rotor blades about center 24 with respect to rotor shaft 18'.

In this embodiment the rotor blades are fixed to the outer ends of pitcharms 92, which are bolted to the outer race of the bearing for gimballedmovement about center 24. Drive plate 82 maintains its horizontalposition as the rotor pivots, but attachment 78, outer rate 74 and theattachment 94, where the pitch spindle is connected to the outer race,pivot about center 24. The rotor shaft is therefore, separated from themovement of the outer bearing race and the components connected to it bythe cone 96 of the bearing, which is made from elastomer and metallaminas having spherical surfaces centered at 24. The deflection of theouter race relative to the inner race causes shear strain in theelastomericc bearing cone.

Rotor torque is carried from the transmission output by rotor shaft 18'to drive plate 82 from which it is transferred by attachment 80 tocoupling 76. The stiff torsional resistance of the coupling transmitsrotor torque to the pitch spindle adjacent attachment and at attachments94 and 98.

What is claimed is:
 1. A rotor for supporting helicopter rotor bladesthat extend radially outward from the rotor comprising:a rotor shaftdriveably connecting the rotor to a power source, mounted for rotationabout a central longitudinal axis; a rotor hub surrounding the axis ofthe rotor shaft and adapted for attachment to the rotor blades; bearingmeans located between the rotor hub and the rotor shaft, defining acenter about which the rotor hub is supported on the rotor shaft; andflexible shaft means coaxial with, connected to and forming an extensionof the rotor shaft, said flexible shaft means also being connected tothe rotor hub for flexibly connecting the rotor shaft to the rotor hubfor pivotal movement about said center and for inflexibly connecting therotor shaft to the rotor hub for torsional movement, wherein the loadpath by which rotor torque is transmitted between the rotor shaft andthe rotor hub through the bearing means is flexible in relation to theload path by which rotor torque is transmitted between the rotor shaftand rotor hub through the flexible shaft means.
 2. The rotor of claim 1wherein the bearing means comprises:an inner race driveably connected tothe rotor shaft defining a spherical surface; an outer race driveablyconnected to the rotor hub defining a spherical surface concentric withthe spherical surface of the inner race; flexible core means locatedbetween the inner race and outer race for resiliently connecting theinner and outer races for movement about said center.
 3. The rotor ofclaim 2 wherein the bearing means comprises multiple pairs of inner andouter races, one pair located on laterally opposite sides of the planethat is perpendicular to the axis of the rotor shaft and passes throughthe bearing center.
 4. The rotor of claim 1 wherein the flexible shaftmeans is made from fiber material directed substantially at a forty-fivedegree angle on opposite sides of and with respect to the axis of theflexible shaft means, said material being supported by a matrix ofpolymeric urethane resin.
 5. The rotor of claim 1 wherein the flexibleshaft means is made from fiber material whose fibers are directedsubstantially angularly biased with respect to the axis of the flexibleshaft means and the fibers are supported by a matrix of polymeric resin.6. The rotor of claim 5 wherein the fibers are supported by resin madefrom urethane.
 7. The rotor of claim 1 wherein the flexible shaft meansis made from fiber material supported by a matrix of polymericurethane-based resin, the composite material formed of said fibermaterial and said resin having a ratio of Young's modules to shearmodules that is variable over the range from 0.001 to 10,000 inaccordance with the angular direction of the fibers with respect to theaxis of an applied axial force-and a shear load.
 8. The rotor of claim 1wherein the torsional stiffness of the flexible shaft means about itsaxis is large compared to its bending stiffness about an axisperpendicular to its longitudinal axis.
 9. The rotor of claim 1 whereinthe torsional stiffness of the flexible shaft means about its axis islarge compared to its bending stiffness about an axis perpendicular toits longitudinal axis and compared to its axial stiffness.
 10. The rotorof claim 4 wherein the resin is made from a urethane polymer from thegroup consisting of MXU-34, MXU-17, and MXU-10.
 11. The rotor of claim10 wherein the fiber material is from the group consisting of S2 fiberglass or E fiber glass.
 12. A rotor for supporting helicopter rotorblades that extend radially outward from the rotor comprising:a rotorshaft driveably connecting the rotor to a power source, mounted forrotation about a central longitudinal axis; a rotor hub surrounding theaxis of the rotor shaft and adapted for attachment to the rotor blades;bearing means located between the rotor hub and the rotor shaft,defining a center about which the rotor hub is supported on the rotorshaft; and flexible shaft means coaxial with, connected to and formingan extension of the rotor shaft said flexible shaft means beingconnected on one side of said center to the rotor shaft and to the rotorhub on the other side of said center for flexibly connecting the rotorshaft to the rotor hub for pivotal movement about said center and forinflexibly connecting the rotor shaft to the rotor hub for torsionalmovement, wherein the load path by which rotor torque is transmittedbetween the rotor shaft and the rotor hub through the bearing means isflexible in relation to the load path by which rotor torque istransmitted between the rotor shaft and rotor hub through the flexibleshaft means.
 13. A rotor for supporting helicopter rotor blades thatextend radially outward from the rotor comprising:a rotor shaftdriveably connecting the rotor to a power source, mounted for rotationabout a central longitudinal axis; a rotor hub surrounding the axis ofthe rotor shaft and adapted for attachment to the rotor blades; bearingmeans located between the rotor hub and the rotor shaft, defining acenter about which the rotor hub is supported on the rotor shaft;flexible shaft means connected to the rotor shaft and rotor hub forflexibly connecting the rotor shaft to the rotor hub for pivotalmovement about said center and for inflexibly connecting the rotor shaftto the rotor hub for torsional movement, said flexible shaft meansextending vertically from a connection to the rotor shaft located belowthe bearing means upward above the bearing means; and a connectingmember located above and connected to the rotor hub and connected to theflexible shaft means above the bearing means.
 14. The rotor of claim 13wherein the flexible shaft, means comprises two concentric coaxialshafts, each shaft connected to the rotor shaft below the bearing meansconnected to the rotor hub above the bearing means.